This invention relates to a compound of molecular formula LixCryMn2xe2x88x92yO4+z, wherein 2.2 less than x less than 4, O less than y less than 2 and zxe2x89xa70, and to the use of this compound as a cathode material in secondary lithium and lithium ion cells.
The impetus for this invention was the recent, great increase in demand for rechargeable batteries that combine high specific and volumetric energy density and with low cost and thermal stability.
A lithium ion cell is a rechargeable electrochemical cell in which the electrochemically active components in both the cathode and the anode are lithium intercalation compounds. The intercalation compound serves as a host structure for lithium ions, which are either stored or released depending on the polarity of an externally applied potential. A lithium ion cell consists of a lithium intercalation cathode with an oxidizing potential and a lithium intercalation anode with a reducing potential. A lithium intercalation material is able to reversibly store and release lithium ions in response to an electrochemical potential. On discharge, in a lithium ion cell, lithium ions move from the anode to the cathode, and thereby, generate an electrochemical current. On charge, energy is consumed in forcing the lithium ions from the cathode to the anode. The greater the difference in the potentials of the cathode and the anode the greater the electrochemical potential of the resulting cell. The larger the amount of lithium which can be reversibly stored in and released from the cathode and the anode, the greater the capacity. The cell""s discharge capacity reflects the time duration for which a cell can deliver a given current. Typical anodes for a lithium ion cell are made from carbonaceous materials such as graphite or petroleum cokes. Typical cathodes are made from transition metal oxides or sulphides. For ease of cell fabrication, lithium ion cells are normally built in the fully discharged state with lithium present only in the cathode and not in the anode. In this state usually both the anode and the cathode materials are air stable. Assembling the cell in the discharged state means that the ultimate capacity of the cell depends on the amount of lithium initially present in the cathode. For example, cathodes based on LiMnO2 have twice the theoretical capacity of cathodes based on LiMn2O4.
Previous reports and patents on cathodes for lithium ion cells have proposed using various mixed oxides of lithium, such as LiCoO2, LiNiO2, LiNixCo1xe2x88x92xO2 and LiMn2O4 as the active material.
The use of phases such as LiMn2xe2x88x92xCrxO4.35 where 0.2 less than x less than 0.4, and LiCrxMn2xe2x88x92xO4 where 0 less than x less than 1, in secondary lithium batteries which have a metallic lithium anode, are also known. See G. Pistoia et al, Solid State Ionics, 58, 285 (1992) and B. Wang et al, Studies of LiCrxMn2xe2x88x92xO4 for Secondary Lithium Batteries, extended Abstract from the Sixth International Meeting on Lithium Batteries, Munster, Germany, May 10-15, 1992. (See also J. Power Sources, 43-44. 539-546 (1992). The materials are described in the latter case as being of a cubic lattice structure. Also in the latter case, additional Li was inserted electrochemically. However, only an additional 0.4 mole equivalents of lithium could be inserted e.g. to provide an oxide of molecular formula Li1.4Cr0.4Mn1.6O4.
Also, a lithium-poor lithium-manganese spinel structure for use in secondary electrochemical cells, having a molecular formula of LiqMxMnyOz where q is 0 to 1.3, is described in U.S. Pat. No. 5,169,736.
More recently, the amount of lithium in such mixed metal oxides has been increased. In our previous U.S. Pat. No. 5,370,949, issued 6 Dec. 1994, a single phase compound of molecular formula Li2CrxMn2xe2x88x92xO4, wherein 0 less than x less than 2, and its use in secondary lithium ion cells is described. These materials were prepared by standard solid state techniques and demonstrated good cycleability with discharge capacities up to 170 mAh/g at low current densities (3.6 mA/g). Higher discharge capacities were found for compositions in which at least half of the transition metal content was manganese. However phases with significantly more manganese than chromium developed a less commercially attractive two-plateau voltage curve on the first or subsequent discharges. Further structural and electrochemical characterization of these cathode materials was published by us in two papers in the Journal of Power Sources (volume 54, pages 205-208, in 1995 and volumes 81-82, pages 406-411, in 1999).
Another related material is described in U.S. Pat. No. 4,567,031 of Riley, issued 28 Jan. 1986. The abstract of this reference describes a mixed metal oxide having the formula LixMyOz where M is at least one metal selected from the group consisting of titanium, chromium, manganese, iron, cobalt and nickel. It is significant that none of the enabled structures include Mn or Cr. Moreover, there is no enabled disclosure of the use of such compounds as cathodes for secondary cells.
Also, the preparation and characterization of materials of compositions Li2CrxMn2xe2x88x92xO4 in which 1.0xe2x89xa6xxe2x89xa61.5 was described by Dahn, Zheng and Thomas (J. Electrochem. Soc., 145, 851-859, Mar. 1998). These materials were made using a sol-gel technique followed by heating in an inert atmosphere to temperatures ranging from 500 to 1100xc2x0 C. In this study a maximum discharge capacity of 150 mAh/g at a low current density (1.5 mA/g) was found for a sample of composition Li2Cr1.25Mn0.75O4 which had been prepared at 700xc2x0 C. The same sample cycled at 15 mA/g showed a capacity of about 137 mAh/g. The materials prepared at 700xc2x0 C. were all found by powder x-ray diffraction to have a layered structure like LiCoO2 with a hexagonal unit cell symmetry. The volume of the crystallographic unit cells varied from 68.6 to 71.9 cubic angstroms per formula unit of Li2CrxMn2xe2x88x92xO4.
U.S. Pat. No. 5,858,324 issued Jan. 12, 1999 discloses a process for preparing a compound of formula LiyCrxMn2xe2x88x92xO4+z where yxe2x89xa72, 0.25 less than x less than 2 and zxe2x89xa70. The experimental examples provided in the disclosure demonstrated the use of these materials as the active cathodes in rechargeable lithium ion electrochemical cells. The LiyCrxMn2xe2x88x92xO4+z samples were prepared as in the J. Electrochem. Soc. (145, 851-859, March 1998) publication from chromium nitrate nonahydrate, manganese (II) acetate tetrahydrate and lithium hydroxide monohydrate by a sol-gel process using ammonium hydroxide as the gelling agent. The gels were heat treated in inert atmosphere at temperatures ranging from 500 to 1100xc2x0 C. to form LiyCrxMn2xe2x88x92xO4+z compounds.
The compounds were characterized by Reitveld refinement of crystallographic unit cell dimensions from powder x-ray diffraction data. The symmetry and space group of the compounds was found to depend on the composition and processing temperature. The structure refinements and chemical analyses of the compositions studied are summarized in Table 1 of the patent (U.S. Pat. No. 5,858,324). Table 1 of the patent (U.S. Pat. No. 5,858,324) summarizes the structure refinements and chemical analyses for the 37 samples studied. Only two of the examples (samples 22 and 23) of LiyCrxMn2xe2x88x92xO4+z have compositions in which y is greater than 2.0. The value of x ranges from 0.5 to 1.5 and the value of z as reported in table 6 is zero.
Only samples prepared at the lowest temperatures, 500 or 600xc2x0 C., have crystallographic unit cell volumes per stoichiometric unit which are substantially smaller than that of LiCrO2 at 69.9 cubic Angstroms. Electrochemical characterization of these samples is not provided in the (U.S. Pat. No. 5,858,324) patent disclosure. The compositions of LiyCrxMn2xe2x88x92xO4+z in examples 22 and 23 listed in table 1 of U.S. Pat. No. 5,858,324 have y equal to 2.2 and x equal to 1.25. The crystallographic unit cell volumes per stoichiometric unit for example 22 which was prepared at 500xc2x0 C. at 68.532 cubic Angstroms is significantly less than the 69.9 to 74.3 cubic Angstroms range provided for the same compounds as described in our previous U.S. Pat. No. 5,370,949. Electrochemical characterization of sample 22 is not provided in the U.S. Pat. No. 5,858,324 patent. The electrochemical evaluation of Example 23 from U.S. Pat. No. 5,858,324 which has a composition of Li2.2Cr1.25Mn0.75O4 and a normalized unit cell volume of 69.81 cubic Angstroms is summarized in Table 6. The discharge capacity of this sample can be compared with that of example 3xe2x80x943 in the same table, which has a composition of Li2Cr1.25Mn0.75O4, a normalized crystallographic unit cell volume of 70.18 cubic Angstroms, and was tested under the same electrochemical conditions. Example 23 demonstrated a discharge capacity of 117 mAh/g only slightly better than that of example 3xe2x80x943 at 106 mAh/g. This small difference in the discharge capacities of the two examples falls within the normal range of variability between cells containing identical cathodes.
The U.S. Pat. No. 5,858,324 patent was also filed as an international PCT application (WO 98/46528 and PCT US98/04940).
A similar material is described in Japanese Kokai 07,272,765, published 20 Oct. 1995. In this reference, the mixed oxide is sintered with vanadium compounds to form a positive electrode material for a secondary cell.
It is an object of the present invention to provide novel lithium oxide materials for use as the active material in cathodes for lithium ion electrochemical cells.
It is another object of the present invention to provide a secondary electrochemical cell of high energy density, whose charge/discharge mechanism is based upon alternating intercalation and deintercalation of Li+ ions in the active materials of the positive and negative electrodes.
It is a further objective of this invention to provide materials useful as cathodes in lithium ion cells that exhibit unusually large charge and discharge capacities.
It is yet another object to provide good chemical resistance to the electrolyte and high cycling stability.
It is a further object of this invention to provide compounds containing greater amounts of lithium. Increased lithium content provides additional paths for lithium ion mobility and increases the theoretical capacity of the cathode.
According to one aspect of the invention, a novel compound of molecular formula I,
xe2x80x83LixCryMn2xe2x88x92yO4+zxe2x80x83xe2x80x83I
wherein 2.2 less than x less than 4.0, 0 less than y less than 2 and zxe2x89xa70, is provided.
While it is preferred that the value of x be in the range of about 2.2 to about 4, it is more preferred that x be in the range of 2.2 to 3.6.
Similarly, while it is more preferred that the value of y be in the middle and lower end of the range, i.e. about 0.1 to about 1.75, useful compounds according to the invention toward the upper end of the range, i.e. up to about 1.9 are included.
In addition, it is preferred that z be  greater than 0, it is more preferable that 0 less than zxe2x89xa62.6.
The compounds of formula I may be further characterised by the crystallographic unit cell volume when indexed in hexagonal symmetry to a Rxe2x88x923 m structure, being smaller than that of LiCrO2 i.e. smaller than 104.9 cubic angstroms, and yet further characterized by the average cation to anion bond distance being smaller than that of LiCrO2. This value is available in the literature.
According to another aspect of the invention, the use of compounds of molecular formula I as active cathode material in secondary lithium ion electrochemical cells is also provided.
According to yet another aspect of the invention, a secondary lithium ion electrochemical cell comprising a lithium intercalation anode, a suitable non-aqueous electrolyte including a lithium salt, a cathode of a compound of formula I as defined above as initial active material, and a separator between anode and cathode is provided.
According to a further aspect of the invention, several processes for making the novel compounds of molecular formula I, are provided. The details of the processes are described below.
The anode of the present invention serves as the recipient for Li+ ions. Accordingly, the anode can be of any intercalation compound, which is capable of intercalating lithium and has an electrode potential sufficiently reducing to provide an adequate cell voltage over a range of lithium intercalation. Specific examples include transition metal oxides such as MoO2 or WO2 [Auborn and Barberio, J. Electrochem. Soc. 134 638 (1987)], the disclosure of which is incorporated herein by reference, transition metal sulfides (see also U.S. Pat. No. 4,983,476) or carbon products obtained by the pyrolysis of organic compounds. As will be apparent hereinafter, various commercially available carbonaceous materials of predetermined structural characteristics have proven useful.
The cathode of molecular formula I as defined above, is an intercalation compound with an electrochemical potential sufficiently positive of the anode to produce a useful overall cell voltage. The greater the potential, the greater the resulting energy density. The cathode generally serves as the initial reservoir of lithium. The capacity of the cell will be limited by the amount of lithium, available for deintercalation, present in the cathode. In most cases, only a proportion of the lithium present, during fabrication of the cathode can be reversibly deintercalated.
The non-aqueous electrolyte of the present invention can be liquid, paste-like or solid. In particular, the electrolyte could be a solid or gelled polymer. Polymers useful for electrolytes in lithium cells include polyvinylidene fluoride (PVDF), with and without co-polymers, and polyethylene oxide (PEO). The electrolyte typically includes a lithium salt in a liquid organic solvent. Lithium salts useful for this purpose include LiAsF6, LiPF6, LiBF4, LiClO4, LiBr, LiAlCl4, LiCF3SO3, LiC(CF3SO2)3, LiN(CF3SO2)2, and mixtures thereof. LiAsF6 should be used with caution due to its toxicity. As a water-free solvent for these salts, there can be used alone or in mixture with others an organic solvent of the group propylene carbonate, ethylene carbonate, 2-methyl tetrahydrofuran, tetrahydrofuran, dimethoxyethane, diethoxyethane, dimethyl carbonate, diethyl carbonate, methyl acetate, methylformate, xcex3-butyrolactone, 1,3-dioxolane, sulfolane, acetonitrile, butyronitrile, trimethylphosphate, dimethylformamide and other like organic solvents. The electrolyte solution can also contain additives such as Crown ethers eg. 12-C-4, 15-C-5, and 18-C-6, or immobilising agents such as polyethylene oxide or inorganic gel-forming compounds such as SiO2, or Al2O3 such as described in U.S. Pat. No. 5,169,736, the disclosure of which is incorporated herein by reference.